1. Field of the Invention
The invention relates to and active discharge electrode.
2. Description of the Prior Art
Discharge electrodes are known as passive discharge electrodes in the form of grounded points or tongues in a plurality of embodiments. In addition, there are also so-called active discharge electrodes which can be connected to a source of high-voltage alternating current. They serve to discharge or eliminate positive and/or negative charges on the surface of preferably fast-moving material webs, like those used for example in gravure printing.
Usually these known active discharge electrodes have at least several rows of needle-shaped individual electrodes arranged parallel to one another, said electrodes being arranged parallel to one another within a row with a point at their free ends and can be connected to a source of high voltage. At least one ground conductor is located along a row of electrodes and preferably parallel thereto.
A study of such known active discharge electrodes has revealed that their efficiency during discharge is relatively poor and leaves a relatively high residual charge on the material web, especially on fast-moving material webs traveling at more than 2 to 3 meters per second. The same is true of plastics with closed surfaces, for example polymer films such as polycarbonate or polyester with a breakdown voltage of more than 4000 volts and a surface resistance of more than 1014 ohms and a specific volume resistance of 1014 ohms per cm.
The invention is an active discharge electrode which permits the highest possible charge in fast-moving material webs and in the above plastics, in which a double charge layer can form.
This goal is achieved in a discharge electrode according to the preamble of the main claim according to the invention by its characterizing features in a surprisingly simple fashion.
Remarkably, it has been found that when a plurality of rows of needle-shaped individual electrons run parallel to one another, the result of the discharge is not improved but on the contrary is even made worse. It has been found according the the invention that a single row of individual electrodes produces the best results. In addition, it has also surprisingly been found that efficiency is even further improved when the ground conductor is completely surrounded by an electrical insulator. Even with fast-moving material webs with a speed on the order of more than 10 meters per second, a maximum possible discharge can be achieved. In the electrode according to the invention, there is also the advantage if the power fails that the row of individual electrodes act as passive discharge electrodes so that at least a minimum of functional ability is retained despite the power failure. This effect, again surprisingly, does not occur when the connections between the ground conductor and the individual electrodes are switched, in other words the ground conductor is connected to a source of high voltage alternating current and the needle-shaped individual electrodes are grounded. According to the invention, the individual electrodes are connected to a voltage form the high voltage alternating current source that is symmetrical with respect to ground.